4-4
4.13 HEX ASCII
Storage in most digital devices is in groups of 8 bits, called bytes. Each byte has a
most-significant nibble (the left most 4 bits) and a least-significant nibble.
To make the best use of the available storage, all possible bit sequences should be
used, so each nibble can have 16 different values (not just the ten of decimal notation).
These 16 values are symbolized by 0-9 and A-F, the hexadecimal code.
The meter transmits almost all data (shown in format statements as <data>) in this
HEX-ASCII form: each byte is broken into its two nibbles, each nibble is given its HEX
symbol, and the ASCII character (table address) for each of those two HEX symbols is
then transmitted (most significant nibble first).
The transmitter and receiver must know whether a number or a non-numerical symbol
is being sent by HEX-ASCII: this is the reason for standard FORMATs in the meter
commands and responses. To illustrate this requirement, if you decode two adjacent
characters as “0110100” (the code for the symbol “4”) and “1000001” (the code for the
symbol “A”), do you print “4A” or do you print the symbol whose hex table address is
4A, the letter “J”? The format statements tell you which is which.
The responses to “V” and “X” commands encode the numerical <data> in HEX-ASCII,
but use decimal (BCD) nibbles (4 bits per decimal digit), storing these two BCD digits
per byte (rather than 8 bits of straight binary). Decoding to decimal is then simplified
for receiving devices such as printers.
“V” and “X” commands also use a single plain ASCII character for each “-”, “.”, and
units-of-measure symbols, in contrast to the “G, P, R or W” commands and responses,
which encode everything in HEX-ASCII, 2 characters to the byte.
4. Definitions
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